Is the Adenosine Receptor Modulation of Histamine‐Induced Accumulation of Inositol Phosphates in Cerebral Cortical Slices Mediated by Effects on Calcium Ion Fluxes?

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Abstract
In this report, we show that under conditions designed to provide an initially uniform incorporation of [3H]inositol into mouse and guinea pig cerebral cortical slices prior to agonist stimulation, the accumulation of 3H‐inositol phosphates (3H‐InsPx, x = 1–4) induced by histamine in mouse and guinea pig cerebral cortical slices increased in a quasilinear manner with increasing added calcium. Raising the ambient calcium ion concentration failed to reduce the adenosine receptor‐mediated inhibition of the histamine‐induced 3H‐InsPx response in mouse cerebral cortical slices. Similarly, the potentiation of the histamine response by adenosine receptor activation in guinea pig cerebral cortical slices was unaffected by lowering the added calcium ion concentration. The presence of the calcium ionophore A23187 (33 μM) produced 3H‐InsPx responses in both mouse and guinea pig cerebral cortical slices, which were not affected by the presence of the stable adenosine analogue 2‐chloroadenosine. A23187 also potentiated the accumulation of 3H‐InsPx induced by histamine in both species. Both the inhibitory and potentiatory modulations of the histamine response by 2‐chloroadenosine in mouse and guinea pig, respectively, were still apparent in the presence of A23187. These results indicate that the histamine‐induced 3H‐InsPx accumulations in both mouse and guinea pig cerebral cortical slices are sensitive to variations in calcium ion concentrations. However, the adenosine receptor modulations of the histamine responses are relatively insensitive to fluctuations in either extra‐or intracellular calcium ion concentrations, and thus cannot be mediated by effects on calcium ion movements.